Combined fuse and current limiting resistor



COMBINED FUSE AND CURRENT LIMITING RESISTOR Filed April 19, 1956 Feb. 10, 1959 E. BERNSTEIN 2 Sheets-Sheet 1 ICE-.1].

INVENTOR. ELLIOT Eerzuswfiwl BY @W 61121, M

* a m IFJ Feb. 10, 1959 E. BERNSTEIN 2,873,327

comsmsn FUSE AND CURRENT LIMITING RESISTOR Filed April 19, 1956 2 Sheets-Sheet 2 ELL/0T Bea/s rE/U INVENTOR.

BY 9% Y United States Patent COMBINED FUSE AND CURRENT LIMITING RESISTOR Elliot Bernstein, Cedarhurst, N. J.

' Application April 19,- 1956, Serial No. 579,385

5 Claims. (Cl. 200-135) and'yet any increase in the current value either beyond a predetermined amount or beyond a predetermined time delay period will result in blowing the fuse.

Standard fuses at present used for home electronic purposes, such as television, will almost invariably blow on a minor momentary surge of high amplitude which is so short in duration as to create no probability that any of the circuits would be damaged. Further, such prior fuses tend to derate under ambient temperatures above normal or when operated close to their maximum rating so that they will cause outages on the occurrence of minor surges, and sometimes even without the occurrence of an overload.

Since standard practice requires that such fuses, particularly in home operated devices, be so arranged'that they require a trained serviceman to replace them such relatively minor momentary surges in energy will frequently result in an outage requiring a service correction at great expense either to the manufacturer who has guaranteedservice or to the consumer who must obtain such service.

Essentially my invention contemplates the utilization of a high temperature co-efllcient wire as a fuse wire, which wire, however, is not melted directly by the increased heat generated therein by increased current; but the wire is used'primarily to provide a resistance which will rise in accordance with the rate of the current. The wire is so wound that it has preferably the minimum necessary contact with any support so that transfer or drawing off of heat by conduction by outside mounting elements is obviated (the ideal condition being a wire which is completely unsupported except at the ends).

Since the wire is of sufficient length to create the desired resistance characteristics, it is usually utilized in coil form or other wound form in order to reduce the size of the fuse to appropriate commercial dimensions.

The high temperature, co-emcient wire has the characteristic that its resistance when cold is at a minimum. A flash-over current heats the Wire to increase the resistance thereby limiting the amplitude of the discharge and limiting the rate of rise of the heat. Fault current when it occurs over a minimum period of time (such time may be measured in seconds or, fractions of seconds will generate sufiicient heat owing to increasing re: sistance of the wire to result in a blowing of the fuse.

My invention further contemplates that the blowing of thefuse will not depend per se on the rupture or melt: ing of the wire when heated, but in order to ensure that the rupture of the Wire and the blowing of the fuse will take place Whenthepredetermined Wire. temperature is reached, I place oneor more beads of a chemica1 compound adapted to decompose rapidly under predeter- 2,873,327 Patented F eb. 10, 1959 2 mined heat conditions on, the wire. By using thechemical bead, I am able to use athicker wire that wi-ll re main relatively cool and, therefore stable in resistance at steady-state currents.

The specific composition of this head will behereinafter described but essentially-it is-acombination of magnesium powder, silicapowder,- barium peroxide powder and a dehydrated nitrocellulose lacquer.

Accordingly utilization o-f the bead in connection with the high temperature co-efficient'wire make it possib le to have a fuse which is-a currentlimiting-resistorthereby tending to limit any flash-over currents while at the same time it is a fuse which will'openthe circuit under appropriate fault conditions.

As previouslypointed out, the effective; temperature co-efiicient of the resistance wire is limitedby any contact with a coil formwhich may draw offor dissipate the heat in the wire. Thus, for instance, my fuse is so arranged that in a case where normal current may be of the order of 200 milli-amperes, aflash-over current which would otherwise increase the load'by amperes will in a milli-second or less be limited to a much smaller rise. Thus, with a basic-resistance of'2-ohms in a usual fuse, anda discharge of 3OO volts, the surge would be 150 amperes. However,- my novel unit, with its. inherent resistance of- 20 ohms, immediately limits the current to 15 amperes and thenresponds to the rate ofrise of'heat and promptly increases resistance to furtherlimit the-surge I However, a fault current of 250' percent (500 milliamperes) lasting over a period of time-say of the-.o rder. of 10 seconds, will result in opening of thecircuit. As the fault current becomes higher, there is a'progressive decrease in time.

I have also found that by windinga- 2( "wire of .0035 on a 1" form where the wire has minimum contact with the form, I.produce a fuse in which the fuse wire has a 20 ohm resistance at normal operating currents of 200 milli-amperes' and that this resistance effectively becomes 150 ohms at a 300 =volt-surge. The wireitself is a standard Wire known to; have increased resistance with increased temperature and is formed-of either iron or nickel, or an alloy'of iron and nickel, or of. tungsten, and is-known in the industry as-Hi Tempco or-Balcon No. 236 -alloy,- .0035 in diameter,

Such awire may be oxidized-for two purposes: (l) for insulation; and (2) to create a thermal barrier in order to increase the. thermal lag.

Essentially therefore my invention combinestwo important concepts in a single-structure. Thearrange ment and mounting of the Wire to decrease to the ab-. solute minimum any thermal lag in the wire so that the increased heat'of the wire in response -to increased currents thereby increasing the resistance will create an automatic current limiting resistor; at thesametime the utilization of the rupturing bead on the wire which will decompose rapidly at predetermined temperatures turns this current limiting resistor into a fuse.

The primary object of'my invention therefore is the provision of a combined current limitingresistorand fuse.

Anotherobject of my invention is the-arrangementcfaeurrent limiting resistor so that theconductive-element thereof will be mounted in such manner-astodimit its physicaland thermally conductive contact to the minimum necessary for actual support of-thewire.

Another-object of my invention is incombination witha wire which responds-thermally to any: increasein current to utilize a fast thermallydecomposing bead ofmaterial on the; wire in order to rupture thewire whena predetermined currentvalueor predetemined time, times current value is reached-1.

. tact with the wire.

Figure 4 is a cross-sectional view of a coil support slightly modified in structure from the coil support of Figure 3 in order to further. decrease the areas of contact between the wire and the coil support.

, Figure 5 is a cross-sectional view of another modified form of coil support.

Figure 6 is another modified form of coil support.

Figure 7 is a. view in expanded perspective showing one way in which the coil and coil support may be housed in a manner which maintains the coil out of contact with the casing.

Figure 8 is a view in perspective of another form of housing for the coil support.

Figure 9 is a view in perspective of another form of housing of the coil support.

Figure 10 is an exterior view of the housing of Figure 9.

Figure 11 is a plan view ofa chassis socket to which the coil in the housing of Figure 10 is to be connected. It should be noted that by appropriate spacing of the prongs of the housing of Figure 10 and of the socket openings in the chassis of Figure 11, only the proper combined current limiting resistor and fuse having properly spaced prongs may be connected in circuit.

Figure 12 is a view of a modified exterior housing construction having a key intended .to limit the insertion of the combined current limiting resistor and fuse into only a particular socket.

Figure 13 is a plan view of a chassis with socket open ings intended to receive only predetermined key formations.

Figure 14 is an expanded view of another modified form of the coil housing. Referring first to Figure l, I have here shown the basic concept of my combined fuse and current limiting resistor in which the wire 10 is a high temperature coeflicient wire of appropriate material. as hereinbefore mentioned in which the resistance increases with increase of heat and the heat of course increases with increase in current.

On this wire is placed a head 11 of material which will decompose rapidly on the incidence of heat of a predetermined value and during decomposition will itself give off sufficient heat to melt the wire. The operative element of the bead 11 is primarily magnesium powder which when decomposing rapidly will burn at a temperature of the order of 2000 F. and thereby immediately melt the wire 10 to break any circuit in which the wire 10 is connected.

Preferably the head is formed of the following materials: 5% magnesium powder; 1.67% SiO powder; 43.44% barium peroxide powder; 50% nitrocellulose lacquer dehydratedand placed on the wire so that it will be heated to initiate decomposition on the occurrence of the predetermined temperature or on the occurrence of a predetermined lower temperature for a predeter mined minimum length of time; I The wire 10 must of course be sufiiciently long to have the proper resistance during surges and to have the desired high temperature co-eflicient in order to create an increased resistance on'temperature rise. Accordingly as shown in Figure 2, the wire 10a is wound as a coil with the bead 11 on one of the turns of the coil.

.As previously pointed out, preferably the coil should be supported only at its ends in order to reduce any drawing off of heat by conduction to the supporting elements so that complete control is achieved by the rate of rise of temperature in the wire itself. Where, however, the wire must be sufficiently long to create correct resistance during surges the coil may require some mechanical support.

For this purpose I provide a coil form 12 as shown in Figure 3, generally of I shaped cross-section around which the high temperature coefficient wire 10b is wound. By the utilization of the I form 12, a minimum of contact of the wire with the form 12 occurs and therefore a minimum amount of heat conduction from the wire to the coil form occurs.

In addition, the coil form 12 provides terminal securement means 13 which may be in the form of eyelets, rivets or any suitable means for securing the ends of the coil of wire 10b and the terminal connections 14, 14.

As shown in Figure 4, even the I shaped coil form of Figure 3 may be modified by adding the longitudinal indentations 15, 15 to the coil form 12a of Figure 4 to decrease the area of contact between the coil form and the wire.

In Figure 3 I have shown three beads 11a on the wire. The decomposition of any one of the heads will result in rupturing of the wire and interruption of the circuit. The utilization of three beads is a safety factor to ensure that at least one of the heads will operate to rupture the wire when the appropriate overcurrent-current value is achieved.

In Figure 5 I have shown in cross-section another modified coil form 12b in which the area of contact between the coil and the form are the longitudinal knife edges 16. This shape may be regarded as the simplest type of form having a star-shaped cross-section.

In Figure 6 I have shown another type of coil form 12c which consists of a rectangular member open on the center around which the coil may be wound. If desired also a groove 17 may be cut in the edges of the form over which the wire is wound in order to reduce further the area of contact betweenthe coil and its form.

In Figure 7 I have shown one method of mounting the coil form 12 in a housing. The housing comprises a casing 20 having a base 21 and appropriate side walls,

said casing being open at the end opposite the base. The cover plate 22 for the opening 23 of the casing has secured thereto in any appropriate fashion the coil form 12d. This coil form is shown as generally having the form of the coil form of Figure 3 but it will be obvious that it may have any of the other cross-sectional shapes already described.

The securement of the coil form'12d to the cover 22 ensures that when the cover 22 is placed over the opening 23 of casing 20, the coil form and the wire thereon will be spaced from the sides of the casing.

An opening 25 in the cover and an opening 26 in the base 21 of the casing will provide means for extending terminals from the coil on the coil form for connection purposes.

In Figure 8 I have shown a modification of the housing of Figure 7 wherein the casing 30 has a bottom wall 31 and an open end wall 32 at the side thereof opposite the bottom wall 31. The coil form 12e is here not secured to the cover but when placed in the casing the end 33' casing 40 having internal end recesses 41, 42 communi-' eating with the open side 43 of the casing serving to pos1t1onthe edges 44 and 45 of the coil form. The re t 3 mainder of the interior of the casing 40 is cut away at 47 and 48 to provide an appropriate spacing between the casing and the coil form. In this case the cover 49 when secured in place by any appropriate means merely en'- closes the casing while the coil form is spaced from the sides of the casing by the internal construction above described.

It will be obvious that any coil form may be used in connection with the casings of Figures 7, 8 and 9 as long as the internal elements of the casing are shaped to correspond to the cross-section or shaping of the coil form.

In Figure 14 I have shown a modified form of coil form and casing wherein the casing 50 is provided with ledges 51, 52 adjacent its base 53 on which the lateral edges 54 and 55 of the wide parts of the coil form 12 may rest with the coil b thereby spaced from the casing. The coil form 12 is provided with connecting terminal prongs 56, 57 to which the coil 10b is connected.

The cover 58 is provided with internal ledges 59, 60 corresponding to the ledges 51, 52 of the casing to cooperate therewith in spacing the coil 10b from the side walls of the casing.

T he cover is also provided with a pair of openings 61, 62 through which the prongs 56, 57 extend to enable the unit when assembled to be connected in circuit. The terminals used in connection with the casings of Figures 7, 8 and 9 may also be in the form of prongs connected to the eyelet or terminal elements 14 of the coil forms in any of those structures.

The prongs 56 and 57 readily extend from the side of the casing as shown in Figure 14 or from the end of the casing as shown in Figures 9 and 10 and may then be used not only to efiect a connection with the appropriate socket but also to key appropriate fuses for insertion only in appropriate sockets.

Thus, as shown in Figure 11, chassis 70 may be provided with one pair of sockets 71 and a second pair of differently spaced sockets 72. If the combined fuse and current limiting resistor of my invention is arranged so that each rating thereof will have a different spacing between the prongs 56, 57 and that each socket for that rating in the chassis of Figure 11 will have each opening matching the spacings of the units to be inserted therein, then only the correct unit may be inserted in a particular pair of socket openings.

In addition, as shown in Figures 12 and 13 my novel unit may be provided with a key 80 cooperating with an appropriate keyway 81 on the chassis 82 of Figure 13. The keyway itself may also carry the circuit connecting elements or may be used in addition to the circuit connecting elements 56, 57 of Figure 10.

The keyways for any particular size or rating of combined fuse and current limiting resistor will be different from the keyways and corresponding keys for every other size and rating. Consequently by matching the keyway 81 and the key 80 for any unit, only the correct unit may be installed in the circuit.

In the foregoing I have described my inventiononly in connection with illustrative embodiments thereof. The various sizes of wire, thickness of wire and length of wire used, and the actual amount of flash material used in the head will be determined by the various values which it is intended to protect.

In my novel bead arrangement, the barium peroxide used therein is the oxidizer, the magnesium powder is the burning element, the silica is used for dehydration purposes, while the nitrocellulose lacquer evaporates oif.

It will be obvious of course that the size of wire determines the flash point. In the construction shown for instance in connection with Figures 3, 7, 8, 9 and 14, for a device intended to protect a 175 milli-ampere circuit with a 500 milli-ampere flash point, I use a 22 ohm resistor made of a nickel-iron alloy (Hi-Tempco or Balco No. 236 alloy), .0035" in diameter with 11 turns on a 1" form or approximately 22" of wire.

This will withstand a discharge of mi. at 300 volts. Three beads are used, one on each end and one in the center. One head will be sufficient but since the beads are placed on the wire by mass production methods, they may therefore vary somewhat in size. The utilization of three beads ensures that there will be at least one good bead.

In view of the many forms which my novel unit may take, I prefer to be bound not by the specific disclosures herein contained but only by the appended claims. It will also be understood that, contemplated within the term coil is any regular or series or regular curvatures or shapings of a length of wire, including zig-zag arrangements, which will make it possible to house or support a relatively long wire in a relatively short housing.

I claim:

l. A combined fuse and current limiting resistor comprising a coil of high temperature coefficient wire and a bead containing magnesium and barium peroxide on said wire, said head having the characteristic of decomposing rapidly under predetermined heat conditions of the wire and itself generating sufiicient heat to melt the wire when said head decomposes.

2. A combined fuse and current limiting resistor comprising a coil of high temperature coefiicient wire and a head on said wire, said head having the characteristic of decomposing rapidly under predetermined heat conditions of the wire and itself generating sufiicient heat to melt the wire when said head decomposes, said head being formed from magnesium powder, silica powder, barium peroxide powder, and nitrocellulose lacquer.

3. A combined fuse and current limiting resistor comprising a coil of high temperature coefficient wire and a bead on said wire, said head having the characteristic of decomposing rapidly under predetermined heat conditions of the wire and itself generating sufficient heat to melt the wire when said head decomposes, said head containing a magnesium salt, a peroxide salt and dehydrating agent.

4. A combined fuse and current limiting resistor comprising a coil of high temperature coefficient wire and a head on said wire, said bead having the characteristics of decomposing rapidly under predetermined heat conditions of the wire and itself generating suflicient heat to melt the wire when said bead decomposes, and an I-section form for said wire coil, said form having a web effecting recesses adjacent a major portion of the coil, said recesses extending between parallel edges of the form supporting the coil and reducing the area of heat conductive contact between the form and the wire of the coil, the coil thus having substantially parallel and linear wire runs, said bead being on one of said runs.

5. A combined fuse and current limiting resistor com prising a coil of resistance wire and a form on which said coil is wound, said form having a recessed portion and said coil having runs of wire disposed across said recessed portion; and a head of material rapidly decomposable under predetermined heat conditions on each of a plurality of said runs.

References Cited in the file of this patent UNITED STATES PATENTS 620,309 Hadaway Feb. 28, 1899 1,818,822 Stafford Aug. 11, 1931 1,861,369 Sundt May 31, 1932 1,143,031 Rapp Jan. 10, 1939 2,294,132 Schuck Aug. 25, 1942 2,773,960 Sundt et a1. Dec. 11, 1956 FOREIGN PATENTS 10,144 Great Britain May 2, 1902 23,369 Great Britain Oct. 11, 1897 52,574 France June 5, 1944 

